The development of thin film flexible lightweight solar cells is of high importance for both terrestrial and space applications. Thin film solar cells use 30–100 times less semiconducting material and are less expensive to manufacture then conventional crystalline silicon cells. Current thin film photovoltaic (PV) research encompasses development of CdTe, Cu(Ga:In)(S:Se)2 (CIS) and thin film silicon based solar cells. One of the most promising technologies lies in the development of polycrystalline thin films, due to their ease of manufacture and importantly, their lightweight structure enables them to achieve higher specific power (W/Kg), than alternative single crystalline devices.
Photovoltaic modules based on ternary chalcopyrite absorber layers, (I-III-VI2; Cu(In:Ga)(S:Se)2) have been the focus of intense investigation. The use of chalcopyrite absorbers are highly appealing since their band gaps correlate well to the maximum photon power density in the solar spectrum for both terrestrial (air mass of 1.5 [AM1.5]), and space applications (AM0), while displaying long term stability and excellent radiation tolerance.
Current methods for depositing ternary crystallite compounds include co-evaporation of multi-source precursors, electrodeposition, reactive-sintering, and flash evaporation, which are often followed by toxic sulphurization/selenization steps at elevated temperatures such as 800° C. Furthermore, under these conditions loss of volatile In/Ga chalcogenides is common. The high temperature requirements of the above methods makes them incompatible with all presently known flexible polyimides, and other polymer substrates. In addition, the use of toxic reagents is a limiting factor.
The use of multi-source inorganic/organometallic precursors in a CVD type process is more appealing due to milder process parameters. However, stoichiometric control of deposited films can be difficult to achieve and film contamination has been reported.
Recently, the synthesis and use of a ternary single source precursor (SSP) having the I-III-VI2 stoichiometry “built in” has been investigated. In 1993, Hirpo and coworkers synthesized [{PPh3}2Cu(SEt)2In(SEt)2], the first known SSP for a ternary I-III-VI2 chalcopyrite material, in this case (CuInS2). Wakgari Hirpo et al., Synthesis of Mixed Copper-Indium Chalcogenolates. Single-Source Precursors for the Photovoltaic Materials CuInQ2 (Q=S, Se), J. Am. Chem. Soc. 1993, 115, 1597–1599. The preparation of hetero binuclear complexes was also reported in the same work. The complexes consisted of tetrahedrally arranged Cu and In centers, with two bridging thiolato and selenolato groups [Eq. 1].                                                         [                                                                    {                                                                  Ph                        3                                            ⁢                      P                                        }                                    2                                ⁢                                                      Cu                    ⁡                                          (                      MeCN                      )                                                        2                                            ]                        +                    +                                    [                                                ln                  ⁡                                      (                    ER                    )                                                  4                            ]                        -                          ⁢                  →          MeOH                ⁢                                                       [                                                                    {                                                                  Ph                        3                                            ⁢                      P                                        }                                    2                                ⁢                                                      Cu                    ⁡                                          (                                              μ                        -                        ER                                            )                                                        2                                ⁢                                                      ln                    ⁡                                          (                      ER                      )                                                        2                                            ]                        +                          2              ⁢              MeCN                                                          [                  Eq          .                                          ⁢          1                ]            
Pyrolysis studies undertaken revealed that the Se derivative could be converted into CuInSe2 at 400–450° C. @ 0.01 mm Hg [Eq.2], but none of the precursors had been evaluated in a thin-film deposition study.                               [                                                    {                                                      Ph                    3                                    ⁢                  P                                }                            2                        ⁢                                          Cu                ⁡                                  (                                      μ                    -                    SeEt                                    )                                            2                        ⁢                                          ln                ⁡                                  (                  SEt                  )                                            2                                ]                ⁢                  →                      400            -                          350              ⁢                                                          ⁢                              C                /                0.01                            ⁢                                                          ⁢              mmHg                                      ⁢                              CulnSe            2                    +                      2            ⁢                          PPh              3                                +                      2            ⁢            EtSeEt                                              [                  Eq          .                                          ⁢          2                ]            
In early studies Nomura et al, reported that an equimolar mixture of [Bui2InSPr] and [Cu(S2CNBu2)2] decomposed to afford CuInS2 powders. Nomura, R.; Kanaya, K; Matsuda, H., Preparation of Copper-Indium-Sulfide thin films by solution Pyrolysis of organometallic sources, Chem. Let., 1988, 1849–1850. On this basis, solution pyrolysis of this mixture dissolved in ρ-xylene was used to deposit thin-film CuInS2 at 350° C. and low pressure onto glass substrates. Film composition was determined by XRD, which showed broad peaks. XRD revealed the ratios of In/Cu and S/Cu decreased with temperature, and a second phase to be present for films deposited at 350° C. Grain size was estimated to be in the range of 50–100 nm as determined by SEM. It was later realized that the equimolar reaction mixture of [Bui2InSPr] and [Cu(S2CNBu2)2] (as used in solution pyrolysis) afforded the single source precursor [Bu2In(SPri)Cu(S2CNPri2)] before decomposing to the chalcopyrite matrix. Analytical and spectral data confirmed that the mixture of [Bui2InSPr] and [Cu(S2CNBu2)2] yielded the above SSP. However, the SSP itself was synthesized in situ as an intermediate to the desired ternary chalcopyrite material starting from the two reagents [Bui2InSPr] and [Cu(S2CNBu2)2] which must be combined in equimolar ratios. A number of analogous ternary CIS precursors were also synthesized by the reaction of alkyl indium thiolates with copper dithiocarbamates [Eq.3].2RIn(SPr)2+2Cu(S2CNR′2)2→2[RIn(SPr)2Cu(S2CNR′2)]+(R′2NCS2)2  [Eq.3]
However, only [Bu2In(SPri)Cu(S2CNPri2)] was successfully implemented for depositing pure CuInS2 by low pressure MOCVD. In the case of [BuIn(SPri)2Cu(S2CNR′2)], tetragonal CuIn5S8 was deposited [Eq.4].                               [                                                    Buln                ⁡                                  (                  SPr                  )                                            2                        ⁢                          Cu              ⁡                              (                                                      S                    2                                    ⁢                                      CNR                    2                    ′                                                  )                                              ]                ⁢                  →                      400            ⁢                                                  ⁢                          C              /              0.6                        ⁢                                                  ⁢            torr                          ⁢                              Cu            5                    ⁢                      lnS            8                                              [                  Eq          .                                          ⁢          4                ]            
In continuing work, Buhro and Hepp were able to demonstrate that [{PPh3}2Cu(SEt)2In(SEt)2] could be utilized in a spray CVD process, for depositing thin-film CuInS2 below 400° C. Hollingsworth, J. A., Hepp, A. F., Buhro, W. E., Spray CVD of Copper Indium Disulfide Films: Control of Microstructure and crystallographic orientation, Chem Vap. Deposition, 1999, 5, 105–108; Hollingsworth, J. A., Buhro, W. E., Hepp, A. F., Jenkins, P. P., Stan, M. A., Spray chemical vapor deposition of CuInS2 thin films for application in solar cell devices, Mat. Res. Soc. Symp. Proc., 1998, 495, 171–176; Harris, J. D., Hehemann, D. G., Cowen, J. E., Hepp, A. F., Raffaelle, R. P., Hollingsworth, J. A., Using single source precursors and spray chemical vapor deposition to grow thin-film CuInS2, Proc. Of the 28th IEEE Photovoltaic Specialists Conference, Anchorage, Ak., 2000, 563–566. Thin films where deposited using a dual solvent system of toluene and dichloromethane, (CH2Cl2) as the carrier solvent. Single phase 112 orientated CuInS2 thin films were deposited from [{PPh3}2Cu(SEt)2In(SEt)2] at a range of temperatures from 300 to 400° C., while at elevated temperatures (>500° C.), CuIn5S8 phase thin films were deposited. RBS EDS and XPS analysis showed that the films were free from any detectable impurities and highly crystalline, thus concluding the precursor decomposes cleanly. During the course of the study, the morphology of the deposited thin films where found to be temperature, and carrier solvent dependent. Films deposited at 300° C. and 350° C. yielded grain size of 400–800 nm, with smaller finer particles of 50–200 nm resident on top. At higher deposition temperature of 400° C., the films consisted of more angular and uniform grain size of approx 200 to 400 nm. Photoluminescence (PL) data and optical transmission measurements confirmed the deposited CuInS2 thin films were direct band gap semiconductors.